1. Field of the Invention
The present invention relates to a terrestrial telescope with a digital camera that uses a means of splitting the optical path so that the optical path goes to the imaging element and the observation optical system.
2. Description of the Prior Art
Terrestrial telescopes having a magnification factor ranging from about 20 to 60 are used extensively for observing wild birds and other fauna. Terrestrial telescopes include those based on a Galilean telescope configuration comprising a positive (convex) lens and a negative (concave) lens that functions as an erecting system, and those based on a Keplerian telescope configuration comprising just a positive (convex) lens, to which are added prisms or other such elements to constitute an erecting system. Both types of telescope enable a user to observe an erect image.
As well as being able to use such telescopes to observe natural flora and fauna, users want to be able to record the images they are seeing. In Japanese Patent Application No. 2002-47304, the present applicant proposed a configuration for a terrestrial telescope with a digital camera that is able to record an observed image.
In the above mentioned Japanese Application the structure of the main optical system except for the observation optical system is similar to that of a single lens reflex camera, and the optical system uses a total-reflection quick-return mirror. Unlike a single lens reflex camera that uses silver-halide film, a single lens reflex digital camera uses a fixed half-mirror to split the optical path so that the beam transmitted by the imaging lens goes to the observation optical system and the imaging element. This makes it possible to continuously use images on the imaging element for display on a monitor, auto-focus processing, calculating exposure, and so forth, and because there is no movable mirror, the configuration can be made simple and low-cost. On the other hand, this configuration reduces the amount of light.
In this respect, Japanese Patent Laid-Open Publication No. 2000-162495 discloses an optical system in which a half-mirror constituted as a quick-return mirror is used to deflect part of the light beam from the subject through the objective lens to the observation optical system and the rest of the beam to the imaging element. In this Publication, the half-mirror is normally located at an observation position at which it deflects part of the subject light beam to the observation optical system and is controlled during imaging to be removed from the imaging optical path. When the half-mirror is in the observation position, the imaging element receives a beam through the half-mirror and photo-electrically converts it to calculate and memorize the focusing position of the objective lens when the half-mirror will be retracted. When the half-mirror is actually retracted to the imaging position during the imaging, the objective lens is moved to the calculated focus position.
The configuration disclosed by Japanese Patent Laid-Open Publication No. 2000-162495 is advantageous in that it avoids light loss during the imaging of the subject and can move the imaging lens to correct a focusing error arising when the half-mirror is retracted. However, it also has drawbacks in that a processor and memory are required for calculating and storing the focus position, which increases the manufacturing cost.
An object of the present invention is therefore to provide a terrestrial telescope with a digital camera that enables the imaging element to continuously perform imaging without loss of light, and in which the focus position of the imaging element can be corrected with a simple and low-cost configuration.
A terrestrial telescope with a digital camera according to the invention comprises a group of objective lenses; an imaging optical system including the group of objective lenses and an imaging element disposed at a position at which an image of a subject is formed by the group of objective lenses; an optical-path-splitting means disposed on the optical path of the imaging optical system between the group of objective lenses and the imaging element so as to be retractable from the optical path of the imaging optical system during imaging; an observation optical system for observing an optical image of the subject via the optical-path-splitting means disposed on the optical path of the imaging optical system; and an imaging position correction means including an optical element that is inserted into the optical axis of the imaging optical system to correct for any change in image-formation position when the optical-path-splitting means is retracted from the optical path of the imaging optical system during imaging.
The optical element can be comprised of a plane glass element of a thickness that corrects for the change in image-formation position arising when the optical-path-splitting means is retracted.
The retraction of the optical-path-splitting means and the insertion of the optical element can be controlled by means of a guide lever that supports the imaging position correction means on one end and the optical-path-splitting means on the other end.
Prescribed optical filtering characteristics can be imparted to the optical element and to the side of the optical-path-splitting means from which light exits to the imaging element.
The plane glass element can be inserted perpendicularly to the optical axis of the imaging optical system.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.